There are various known types of fuel cells. One form of fuel cell that is currently believed to be practical for usage in many applications is a fuel cell employing a proton exchange membrane (PEM). A PEM fuel cell enables a simple, compact fuel cell to be designed, which is robust, which can be operated at temperatures not too different from ambient temperatures and which does not have complex requirements with respect to fuel, oxidant and coolant supplies.
Conventional fuel cells generate relatively low voltages. In order to provide a useable amount of power, fuel cells are commonly configured into fuel cell stacks, which typically may have 10, 20, 30 or even 100's of fuel cells in a single stack. While this does provide a single unit capable of generating useful amounts of power at usable voltages, the design can be quite complex and can include numerous elements, all of which must be carefully assembled.
For example, a conventional PEM fuel cell requires two flow field plates, an anode flow field plate and a cathode flow field plate. A membrane electrode assembly (MEA), including the actual proton exchange membrane is provided between the two plates. Additionally, a gas diffusion media (GDM) is provided, sandwiched between each flow field plate and the proton exchange membrane. The gas diffusion media enables diffusion of the appropriate gas, either the fuel or oxidant, to the surface of the proton exchange membrane, and at the same time provides for conduction of electricity between the associated flow field plate and the PEM.
This basic cell structure itself requires two seals, each seal being provided between one of the flow field plates and the PEM. Moreover, these seals have to be of a relatively complex configuration. In particular, as detailed below, the flow field plates, for use in the fuel cell stack, have to provide a number of functions and a complex sealing arrangement is required.
For a fuel cell stack, the flow field plates typically provide apertures or openings at either end, so that a stack of flow field plates then define elongate channels extending perpendicularly to the flow field plates. As a fuel cell requires flows of a fuel, an oxidant and a coolant, this typically requires three pairs of ports or six ports in total. This is because it is necessary for the fuel and the oxidant to flow through each fuel cell. A continuous flow through ensures that, while most of the fuel or oxidant as the case may be is consumed, any contaminants are continually flushed through the fuel cell.
The foregoing assumes that the fuel cell would be a compact type of configuration provided with water or the like as a coolant. There are known stack configurations, which use air as a coolant, either relying on natural convection or by forced convection. Such cell stacks typically provide open channels through the stacks for the coolant, and the sealing requirements are lessened. Commonly, it is then only necessary to provide sealed supply channels for the oxidant and the fuel.
Consequently, each flow field plate typically has three apertures at each end, each aperture representing either an inlet or outlet for one of fuel, oxidant and coolant. In a completed fuel cell stack, these apertures align, to form distribution channels extending through the entire fuel cell stack. It will thus be appreciated that the sealing requirements are complex and difficult to meet. However, it is possible to have multiple inlets and outlets to the fuel cell for each fluid depending on the stack/cell design. For example, some fuel cells have 2 inlet ports for each of the anode, cathode and coolant, 2 outlet ports for the coolant and only 1 outlet port for each of the cathode and anode. However, any combination can be envisioned.
For the coolant, this commonly flows across the back of each fuel cell, so as to flow between adjacent, individual fuel cells. This is not essential however and, as a result, many fuel cell stack designs have cooling channels only at every 2nd, 3rd or 4th (etc.) plate. This allows for a more compact stack (thinner plates) but may provide less than satisfactory cooling. This provides the requirement for another seal, namely a seal between each adjacent pair of individual fuel cells. Thus, in a completed fuel cell stack, each individual fuel cell will require two seals just to seal the membrane electrode assembly to the two flow field plates. A fuel cell stack with 30 individual fuel cells will require 60 seals just for this purpose. Additionally, as noted, a seal is required between each adjacent pair of fuel cells and end seals to current collectors. For a 30 cell stack, this requires an additional 31 seals. Thus, a 30 cell stack would require a total of 91 seals (excluding seals for the bus bars, insulator plates and endplates), and each of these would be of a complex and elaborate construction. With the additional gaskets required for the bus bars, insulator plates and endplates the number reaches 100 seals, of various configurations, in a single 30 cell stack.
Commonly the seals are formed by providing channels or grooves in the flow field plates, and then providing prefabricated gaskets in these channels or grooves to effect a seal. In known manner, the gaskets (and/or seal materials) are specifically polymerized and formulated to resist degradation from contact with the various materials of construction in the fuel cell, various gasses and coolants which can be aqueous, organic and inorganic fluids used for heat transfer. However, this means that the assembly technique for a fuel cell stack is complex, time consuming and offers many opportunities for mistakes to be made. Reference to a resilient seal here refers typically to a floppy gasket seal molded separately from the individual elements of the fuel cells by known methods such as injection, transfer or compression molding of elastomers. By known methods, such as insert injection molding, a resilient seal can be fabricated on a plate, and clearly assembly of the unit can then be simpler, but forming such a seal can be difficult and expensive due to inherent processing variables such as mold wear, tolerances in fabricated plates and material changes. In addition custom made tooling is required for each seal and plate design.
An additional consideration is that formation or manufacture of such seals or gaskets is complex. There are typically two known techniques for manufacturing them.
For the first technique, the individual gasket is formed by molding in a suitable mold. This is relatively complex and expensive. For each fuel cell configuration, it requires the design and manufacture of a mold corresponding exactly to the shape of the associated grooves in the flow field plates. This does have the advantage that the designer has complete freedom in choosing the cross-section of each gasket or seal, and in particular, it does not have to have a uniform thickness.
A second, alternative technique is to cut each gasket from a solid sheet of material. This has the advantage that a cheaper and simpler technique can be used. It is simply necessary to define the shape of the gasket, in a plan view, and to prepare a cutting tool to that configuration. The gasket is then cut from a sheet of the appropriate material of appropriate thickness. This does have the disadvantage that, necessarily, one can only form gaskets having a uniform thickness. Additionally, it leads to considerable wastage of material. For each gasket, a portion of material corresponding to the area of a flow field plate must be used, yet the surface area of the seal itself is only a small fraction of the area of the flow field plate.
A fuel cell stack, after assembly, is commonly clamped to secure the elements and ensure that adequate compression is applied to the seals and active area of the fuel cell stack. This method ensures that the contact resistance is minimized and the electrical resistance of the cells are at a minimum. To this end, a fuel cell stack typically has two substantial end plates, which are configured to be sufficiently rigid so that their deflection under pressure is within acceptable tolerances. The fuel cell also typically has current bus bars to collect and concentrate the current from the fuel cell to a small pick up point and the current is then transferred to the load via conductors. Insulation plates may also be used to isolate, both thermally and electrically, the current bus bars and endplates from each other. A plurality of elongated rods, bolts and the like are then provided between the pairs of plates, so that the fuel cell stack can be clamped together between the plates, by the tension rods. Rivets, straps, piano wire, metal plates and other mechanisms can also be used to clamp the stack together. To assemble the stack, the rods are provided extending through one of the end plates. An insulator plate and then a bus bar (including seals) are placed on top of the endplate, and the individual elements of the fuel cell are then built up within the space defined by the rods or defined by some other positioning tool. This typically requires, for each fuel cell, the following steps:
(a) placing a seal to separate the fuel cell from the preceding fuel cell;
(b) locating a flow field plate on the seal;
(c) locating a seal on the first flow field plate;
(d) placing a GDM within the seal on the flow field plate;
(e) locating a membrane electrode assembly (MEA) on the seal;
(f) placing an additional GDM on top of the MEA;
(g) preparing a further flow field plate with a seal and placing this on top of the membrane electrode assembly, while ensuring the seal of the second plate falls around the second GDM;
(h) this second or upper flow field plate then showing a groove for receiving a seal, as in step (a).
This process needs to be repeated until the last cell is formed and it is then topped off with a bus bar, insulator plate and the final end plate.
It will be appreciated that each seal has to be carefully placed, and the installer has to ensure that each seal is fully and properly engaged in its sealing groove. It is very easy for an installer to overlook the fact that a small portion of a seal may not be properly located. The seal between adjacent pairs of fuel cells, for the coolant area, may have a groove provided in the facing surfaces of the two flow field plates. Necessarily, an installer can only locate the seal in one of these grooves, and must rely on feel or the like to ensure that the seal properly engages in the groove of the other plate during assembly. It is practically impossible to visually inspect the seal to ensure that it is properly seated in both grooves.
As mentioned, it is possible to mold seals directly onto the individual cells. While this does offer an advantage during assembly when compared to floppy seals, such as better tolerances and improved part allocation, it still has many disadvantages over the technique of the present invention namely, alignment problems with the MEA, multiple seals and molds required to make the seals and more steps are required for a completed product than the methods proposed by the present invention.
Thus, it will be appreciated that assembling a conventional fuel cell stack is difficult, time consuming, and can often lead to sealing failures. After a complete stack is assembled, it is tested, but this itself can be a difficult and complex procedure. Even if a leak is detected, this may initially present itself simply as an inability of the stack to maintain pressure of a particular fluid, and it may be extremely difficult to locate exactly where the leak is occurring, particularly where the leak is internal. Even so, the only way to repair the stack is to disassemble it entirely and to replace the faulty seal. This will result in disruption of all the other seals, so that the entire stack and all the different seals then have to be reassembled, again presenting the possibility of misalignment and failure of any one seal.
A further problem with conventional techniques is that the clamping pressure applied to the entire stack is, in fact, intended to serve two quite different and distinct functions. These are providing a sufficient pressure to ensure that the seals function as intended, and to provide a desired pressure or compression to the gas diffusion media, sandwiched between the MEA itself and the individual flow field plates. If insufficient pressure is applied to the GDM, then poor electrical contact is made; on the other hand, if the GDM is over compressed, flow of gas can be compromised. Unfortunately, in many conventional designs, it is only possible to apply a known, total pressure to the overall fuel cell stack. There is no way of knowing how this pressure is divided between the pressure applied to the seals and the pressure applied to the GDM. In conventional designs, this split in the applied pressure depends entirely upon the design of the individual elements in the fuel cell stack and maintenance of appropriate tolerances. For example, the GDM commonly lie in center portions of flow field plates, and if the depth of each center portion varies outside acceptable tolerances, then this will result in incorrect pressure being applied to the GDM. This depth may depend to what extent a gasket is compressed also, affecting the sealing properties, durability and lifetime of the seal.
For all these reasons, manufacture and assembly of conventional fuel cells is time consuming and expensive. More particularly, present assembly techniques are entirely unsuited to large-scale production of fuel cells on a production line basis.